The invention relates to optical elements for scanning systems, and in particular relates to mirrors for use in multi-directional scanners that are used to position a light beam relative to a target.
Light scanning systems may be used for a wide variety of purposes, such as laser imaging, confocal microscopy, marking, laser drilling, semiconductor processing, and other material processing. For example, multi-directional scanners typically include a plurality of mirrors, each of which is oriented in a different axis. In particular, X-Y scanners may include an X axis mirror that may be rotated with respect to a first axis, and a Y axis mirror that may be rotated with respect to a second axis that is orthogonally disposed to the first axis. Generally, the X axis mirror direct the laser beam across a first range, and the Y axis mirror (which is positioned such that the laser beam contacts the Y axis mirror across the first range) re-directs the laser beam from the X axis mirror across a second range onto a laser application surface. By manipulating the rotational positions of the X axis and Y axis mirrors, it is possible to cover a wide area of laser application surface.
The X and Y axis mirrors in an X-Y scanner must experience large accelerations, and must move with sufficient precision to permit accurate scanning for certain applications such as laser imaging. The maximum speed with which the mirrors may be driven is determined by the maximum rate at which the heavier (typically larger) mirror may be smoothly accelerated. The maximum achievable acceleration of a mirror is essentially limited by the resonant frequency of the mirror since driving the mirror any harder results in imprecise movement. Generally, the larger mirror has a lower resonant frequency and a slower maximum acceleration.
To maximize the rate of acceleration of a mirror, the inertia of the mirror should be as low as possible and the mirror should also be as stiff as possible so that the mirror responds rapidly and precisely to a drive signal. Unfortunately, there is a trade-off between minimizing the weight, and thus, the inertia, of the mirror and maximizing the stiffness of the mirror. The speed and acceleration with which the mirrors may be driven determines how quickly the system may operate to produce a maximum number of, for example, desired marks, images or drill holes per second. If a mirror is driven at an acceleration that excites the resonant frequency of the mirror, then distortion will result.
In current industrial practice, mirrors are typically made of glass, silicon, quartz, and Beryllium. Glass, silicon and quartz all have relatively similar characteristics regarding inertia, and resonant frequency. Beryllium is used in mirrors that must exhibit the highest resonant frequencies and lowest moments of inertia, regardless of cost. Homogenous materials may be characterized as having some density, typically expressed in units of Kg/m3, and a modulus of elasticity or Young""s modulus, typically expressed in GPa or 109 N/m2. The ratio of modulus/density is called the specific stiffness of a material. The higher the specific stiffness, the higher the resonant frequency of a given shape. Alternatively, use of a material with a higher specific stiffness allows a lower mass, and thus lower moment of inertia structure to be designed for a given required absolute stiffness. Beryllium has a specific stiffness of 155.3 106 Nm/kg, vs 31.7 106 Nm/kg for glass, a ratio of about 5:1. Thus, we would expect that for similarly shaped mirrors, one fabricated from Beryllium would have a significantly higher resonant frequency than would one fabricated from glass.
Additional attributes required of a scanning mirror include good optical qualities, such as surface finish, low cost, and low toxicity. Glass and quartz exhibit all of these properties, while Beryllium exhibits few of them. Beryllium is typically used, therefore, when system speed is a primary consideration. Alternatives to Beryllium for fabricating high speed scanning mirrors have been a subject of considerable investigation. One fruitful focus as been on composite structuresxe2x80x94those fabricated of multiple materials, which, together, can exhibit properties superior to those of the component materials.
For example, U.S. Pat. Nos. 4,842,398 and 6,176,588 disclose mirrors formed of composite structures that include honeycomb matrix substrates that are disclosed to be to relatively light weight. U.S. Pat, No. 4,451,119 discloses a composite mirror formed of a three dimensional woven structure of carbon fibers onto which a coating of silicon carbide is applied, followed by a coating of silicon dioxide and then glass, which is polished and then coated with a reflective coating. The ""119 patent discloses that the carbon fiber/silicon carbide bond, the silicon carbide/carbon dioxide bond, and the carbon dioxide/glass bond all provide strong adhesion. The use of the multiple layers, however, generally requires multiple processing steps.
U.S. Pat. No. 5,110,422 discloses a mirror that includes a solid carbon-based substrate onto which a porous pre-deposit material is applied followed by a metal that is deposited onto the porous pre-deposit material. The ""422 patent recognizes that it is difficult to adhere metal to graphite, and discloses that the pre-deposit material provides a metal adhesive by which the metal may be joined to the substrate. The pre-deposit material is disclosed in the ""422 patent to be carbon or silicon carbide, and is disclosed to be deposited onto the substrate by pyrolytic deposition (at 600xc2x0 C., 700xc2x0 C., 1,100xc2x0 C. or 1,150xc2x0 C.), or by chemical vapor deposition. The metal is disclosed in the ""422 patent to be any of cerium, cobalt, chromium, iron, hafnium, iridium, lanthanum, manganese, molybdenum, nickel, osmium, palladium, platinum, rhodium, ruthenium, silicon, tantalum, thorium, titanium, tungsten or uranium, and to be deposited onto the porous pre-deposit material by electrolytic deposition (at 45xc2x0 C. or 50xc2x0 C.), by autocatalytic chemical deposition (at 90xc2x0 C.), by electrophoretic deposition, or by cementation of a metal foil. The processes disclosed in the ""422 patent also require multiple processing steps to apply successive deposits of layers.
There is a need for a scanning system optical element that provides improved stiffness and relatively low inertia.
There is also a need for a scanning system optical element that provides relatively low inertia and a relatively high degree of stiffness, and is economically and efficiently produced with a minimum number of processing steps.
There is further a need for an optical element for a scanning system that provides excellent specific stiffness and is economical and efficient to produce. As used herein, the term specific stiffness with respect to a composite means the ratio of the modulus of elasticity of the composite to the density of the composite, the composite, of course, having a particular shape.
An optical element is disclosed for use in a scanning system. The optical element comprises a carbon-based substrate having a first specific stiffness, and a titanium carbide coating having a second specific stiffness. The carbon-based substrate and the titanium carbide coating form a composite having a desired shape. The composite has a third specific stiffness that is greater than the first specific stiffness and greater than the second specific stiffness.